The invention relates to laser bar code readers, an particularly to readers that scan a laser beam in a sequence of more than one scan line.
Laser bar code readers have been in increasing use, including the slot scanners in supermarkets wherein a single beam is directed by moving mirrors over a sequence of a number of scan lines, often several non-parallel pairs of scan lines.
Hand held laser bar code readers are also in wide use, particularly in warehouses for inventory control, identification of goods, etc.
A characteristic of these multiple-line scanners is the tendency in the reading of bar codes for the return signal to vary depending on which scan line reads the code, due largely to aperture effects on the strength of return beam. The sensitivity of the received light depends on the angle of the beam and the angle of the label. The angle of the beam is inherent in the scanning mechanism, and varies with different slots. The problem is particularly acute with shallow, low-profile slot scanners, which may include critically steep angles on some scan lines.
Also affecting the received signal strength is the angle at which the surface bearing the bar code is held at the instant of reading in a supermarket slot scanner, as well as variations in reflected light and other ambient light conditions.
Little can be done to compensate for gross variations in the angle of the product and in light conditions, but a much greater uniformity in the return signals can be acheived by addressing the problem of the varying strength corresponding to different scan lines.
Automatic gain control generally is not practical in this situation because response time must be extremely fast. Automatic gain control is a following system, but what is needed is to predict the signal strength differences due to scan position before they occur.
It is therefore an object of the present invention to predict and compensate for such variations through a simple and efficient system for adjusting the output signal strength of a bar code reader in accordance with the position of the beam in the scanning sequence at the time a bar code is read.